Defect inspection of semiconductor wafers and reticles (masks) for IC manufacturing is an accepted production process for yield enhancement. The information obtained from a wafer defect inspection tool can be used to flag defective dies for repair, or improve wafer processing parameters. Since a single mask can be used in production of hundreds or even thousands of wafers, therefore, an undetected error or defect in the mask can cause significant loss in yield.
Mask defects are commonly divided into two different categories: hard and soft defects. A defect is called hard defect if it is not possible to be removed by a cleaning process. Added or missing features in the chrome, phase shifter, pindots, scratches, bubbles as well as pinholes fall in this category. A defect is called a soft defect if it can be removed by a cleaning process. Particles, stains, contaminations such as crystals, and residual materials are soft defects. All the hard defects need to be located, fixed, and/or corrected in processes. Although soft defects are removable with a cleaning process, however, accidentally deposited particles on the reticle over a critical size may result in a fatal defect in every die of the wafer, corresponding to zero yield. Therefore in conventional lithography or optical defect inspection process, the reticle is protected with pellicle on the backside to prevent soft defects deposition during process.
In the Extreme Ultra-Violet (EUV) lithography process, this technology is based on 13.5 nm wavelength. However because of high material absorption of this wavelength, EUV reticles do not have a protective pellicle in proximity of the surface to prevent particles from accidentally falling on the pattern area especially during handling and exposure. Stringent specifications are required to ensure that practically no particle is present at the time of exposure.
In the electron beam inspection practice, no matter inspecting conventional wafers, reticles or EUV masks, no protective pellicle can be used. Therefore, preventing soft defects from contaminating the inspected objective surface during inspection practice should be considered seriously.
The present invention restructures the charged particle system to avoid gravitational settling on inspected surface, thereafter meeting the stringent specification for the advanced defects inspection and review.